This invention relates to an adhesive suitable for vinyl chloride-coated steel plates.
Owing to the high processability and good design properties, vinyl chloride-coated steel plates have been widely employed in the fields of household appliances and building materials.
In general, vinyl chloride-coated steel plates are produced by applying a vinyl chloride coating on the surface of a steel plate and then hardening the coating to give a resin film, or by adhering a vinyl chloride film to a steel plate. In these processes, an adhesive is used to adhere the vinyl chloride film or the resin film to the steel plate.
Since adhesives to be used in vinyl chloride-coated steel plates should be excellent in adhesiveness and heat resistance, a number of studies have been made on adhesives usable for these purposes.
For example, JPB S59-37034 discloses an adhesive for vinyl chloride-coated steel plates, which comprises an acrylic resin and an epoxy resin. On the other hand, JPA S58-179274 discloses an adhesive for vinyl chloride-coated steel plates, which comprises an acrylic resin, an epoxy resin and a phenoxy resin. In these cases, the adhesives are applied on steel plates and then hardened with the use of hardeners, thus elevating the adhesive force between the steel plates and vinyl chloride.
Moreover, JPA S58-179274 discloses an adhesive containing a flexibility-imparting agent (nitrile rubber, etc.) to improve the impact resistance at low temperatures.
However, the above-described adhesives suffer from a problem that the vinyl chloride coating films tend to peel off due to the insufficient solvent resistance thereof. To improve the solvent resistance of an adhesive, it has been a practice to elevate the extent of hardening of the adhesive. In the case of the above-described adhesives, however, there arises a problem that the flexibility is lowered with an increase in the extent of hardening, thereby causing the deterioration in the processability and low temperature impact resistance.
Moreover, there is an additional problem that combustible organic compounds are vaporized in the drying step and thus cause environmental pollution or exert undesirable effects on the human body.
In the prior art, therefore, studies have been made to develop adhesives exerting no undesirable effect on the environment, as shown in JPA H1-130764, etc. For example, there has been developed an adhesive for vinyl chloride-coated steel plates wherein a water-dispersible resin is used. However, this water-dispersible resin is an acrylic resin which has a poor solvent resistance, in addition to the defect of peeling off frequently.
An object of the present invention, which has been conceived to solve the above-described problems encountering in the prior art, is to provide an adhesive which is excellent in solvent resistance and low temperature impact resistance and can achieve good adhesion between vinyl chloride and steel plates.
Another object of the present invention is to provide an organic solvent-free adhesive for vinyl chloride-coated steel plates.
The present inventors considered that the poor flexibility of the conventional adhesives for vinyl chloride-coated steel plates might be caused by the fact that these adhesives contained, as the main components, acrylic resins/epoxy resins, acrylic resins/epoxy resins/phenoxy resins, etc.
Thus, the present inventors paid their attention to polyurethane resins having high flexibility and conducted intensive studies thereon. As a result, they have found that the addition of a urethane resin having a polycarbonate structure, among polyurethane resins, to a hardening resin makes the whole adhesive flexible and imparts a good solvent resistance thereto.
When the flexibility is imparted to an adhesive, the solvent resistance of the adhesive is generally deteriorated. On the contrary, the flexibility of an adhesive is lowered in association with improvement in the solvent resistance thereof. However, the present inventors have found out that both of these contradictory requirements can be satisfied each at a practically acceptable level by regulating the content of the urethane resin in such an adhesive to a specific range.
The present inventors further paid their attention to the fact that when the hardening resin and the polyurethane resin, i.e., the main components of the adhesive as described above, were each in the form of an aqueous dispersion, no organic solvent was necessary.
The present invention has been completed based on these findings. Namely, the invention as set forth in claim 1 relates to an adhesive comprising as the main component a hardening resin which is characterized by containing an urethane resin having a polycarbonate structure.
As an example of this adhesive, citation can be made of one wherein said urethane resin is a product obtained by reacting a polyol with an isocyanate compound and said polyol has a polycarbonate structure, as in the invention as set forth in claim 2.
The content of said urethane resin in the adhesive may range from 5% by weight to 35% by weight, as in the invention as set forth in claim 3.
Moreover, it is preferable that said hardening resin and said urethane resin are each in the form of an aqueous dispersion, as in the invention as set forth in claim 4.